Polymeric materials have become increasingly important in the food packaging industry because of their lighter weight, lower cost, and easier processing as compared to the conventional materials such as glass, steel, and aluminum. The barrier properties of the polymeric materials against the food flavor and aroma constituents at different conditions are critical to the suitability of such applications. Generally speaking, there are two major channels available for the losses of the flavor and aroma components to the packaging materials. One is the transmission loss due to the permeation of the flavor/aroma molecules across the polymeric films to the environment, and the other is the sorption loss due to the solubility of the flavor and aroma molecules in the polymer. The permeation rate or permeability at normalized conditions depend on both solubility and diffusivity of the molecules of the polymeric films. Both of the losses have an impact on the shelf life of the packaged food.
Since useful packaging materials must exhibit very low solubility and diffusivity, the measuring techniques must have high sensitivity. In this regard, several techniques have been reported in the literature. For example, an electron impact mass spectrometric technique with high sensitivity and specificity for the permeation rate measurements was reported in "Mass Spectrometric Studies of Diffusion and Permeability in Polymer Films", by R. L. Bredeweg and V. J. Caldecourt, The Fourteenth Annual Conference on Mass Spectrometry and Allied Topics, Paper 29, Dallas, TX, May 22-27, 1966. However, this technique requires that the permeating side of a membrane be exposed to high vacuum for mass spectrometric analysis. This requirement may alter the permeation characteristics of a membrane due to possible physical changes resulting from the vacuum seal and support.
An apparatus based upon flame ionization techniques was discussed in "Measurement of Odor Permeability of Polypropylene Packaging Films at Low Odorant Levels", by M. G. R. Zobel, Polm. Testing, 3 (1982) 133. There it was reported that measurements of permeation rates were capable of being made down to about 3.times.10.sup.-12 Kg m.sup.-2 . sec.sup.-1. However, because of the nature of flame ionization, the technique is incapable of measuring the permeation rates of mixtures.
Two additional techniques based upon atmospheric pressure ionization mass spectrometry and photoionization were discussed in "Atmospheric-Pressure Ionization Mass Spectrometric and Photoionization Techniques for Evaluation of Barrier Film Permeation Properties", by V. Caldecourt and J. C. Tou, J. Memb. Sci., 29 (1986). While these techniques provided comparable sensitivity to that of the flame ionization technique identified above, the photoionization technique did present another drawback. Specifically, the photoionization detector tends to be coated by the photodecomposition products from the permeating molecules resulting in a sensitivity decrease. Additionally, the atmospheric pressure ionization mass spectrometric technique suffers from its inherent narrow dynamic range. Moreover, judgment and further validation would occasionally complicate the operation.
Therefore, the need still exists for a general and reliable technique for the evaluation of the flavor/aroma permeation across a polymer film with its back side at ambient pressure.
Accordingly, it is a principal objective of the present invention to provide a mass spectrometric based system for measuring aroma/flavor permeation rates across polymer films which will be highly sensitive and reliable.
More specifically, it is an objective of the present invention to provide a mass spectrometric based system which is capable of determining permeability, diffusivity and solubility of aroma/flavor constituents of polymeric films with both the feed side the and permeating side of the film at or about ambient pressure.
It is an additional objective of the present invention to provide a permeation measurement device which is capable of achieving parts per billion sensitivity to organic molecule containing vapors with feed concentrations down to the parts per million level.
It is another objective of the present invention to provide a permeation measurement device which is capable of determining the diffusivity and solubility of films where these coefficients cannot be adequately determined from the fast rising transient portion of the permeation curve.
It is a further objective of the present invention to provide a permeation cell and film holder arrangement which enables a film sample to be removed and reinserted into the cell as often as desired with exactly the same alignment.
It is yet an additional objective of the present invention to provide a permeation cell with a film holder which prevents stresses, irregularities or fingerprints from being introduced into the film sample.
It is yet another objective of the present invention to provide a permeation measurement device which permits the exposure of polymer films to gaseous mixtures containing known concentrations of organic molecules under controlled temperature, pressure, and humidity conditions.
It is still another objective of the present invention to provide a permeation measurement device which is capable of independently controlling vapor composition and humidity on each side of the polymer film sample.
To achieve the foregoing objective, the present invention generally comprises a system for measuring the permeation rate of organic molecules across a polymer film, which includes a membrane chamber, an electron impact mass spectrometer and a flow-through hollow fiber probe for providing an interface between the membrane chamber and the mass spectrometer. The membrane chamber supports the polymer film sample to be tested, and permits the sample to be exposed to a vaporous fluid containing a known concentration of organic molecules under substantially atmospheric pressure and controlled environmental conditions on both the upstream and downstream sides of the sample.
The membrane chamber features a unique permeation cell and sample holder arrangement which substantially enhances the operation of the permeation rate measurements. Specifically, the polymer film sample is held between a pair of adhesive backed cards for maintaining a permanent desirable alignment of the sample, as well as permitting convenient placement of the sample into the permeation cell and later storage of the sample. The permeation cell includes a pair of metal disks between which the sample holder assembly is positioned, and fluid pressure operated cylinders for moving the disks into and out of a sealing engagement with the sample. One of the metal disks is formed to provide an upstream cavity, while the other metal disk is formed to provide a downstream cavity. Each of the metal disks is also provided with a pair of ports, so that separate fluids may be swept across each side of the sample.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiments which makes reference to the following set of drawings in which: